Many electronic devices, such as computers, personal digital assistants, cellular telephones, digital cameras and similar systems and devices include processors and memory. The memory is used to store computer programs to be executed by the device and/or data operated on by the processors to achieve the functionality of the device. Many devices and systems require that this information be retained in permanent storage non-volatile medium so that the data and computer programs is not lost when power is removed.
Flash memory (or Flash RAM) is an example of a non-volatile memory device. Flash memory devices use a memory cell transistor with a floating gate structure. The typical memory cell in a flash memory device comprises an access transistor and a storage element, such as a floating gate. Data in the flash memory device are programmed or erased by accumulation or depiction of charge, respectively, across a thin insulating film between a substrate and a storage element (e.g., floating gate). Programming of the memory cells occurs by applying a sufficient voltage difference to the transistors to cause excess electrons to accumulate on the storage element. Erasure of the memory cell is done by applying a voltage difference that causes the charge on the storage element to be extracted.
The voltages used for writing and erasing a flash memory cell are necessarily higher than those used for reading since the information (charge) stored in the cell must be stable over repeated read operations, time and temperature. Typical write and erase voltages are greater than the general supply voltages used by modern microprocessors, volatile memories and transistor—transistor-logic (TTL). When the voltage used for write and erase is also required by other components in a system, the provision of an external voltage source is not an additional cost of using a flash memory. However, when the write and erase voltage is not required by another component, the addition of an internal voltage converter to the flash memory device becomes attractive in order to avoid adding another system voltage source that is useful only to the flash memory. This is particularly true for battery-powered devices with incorporated flash memory.
A common type of voltage converter is the charge pump. Charge pumps are well suited to integration on a semiconductor substrate since they do not require inductors. Charge pumps are usually implemented as an array of capacitors coupled by switches. The capacitor array is driven by a switched low voltage DC source to produce a multiplied DC output. The switching elements are usually diodes or transistors. Examples of charge pumps are the Cockcroft-Walton multiplier and Dickson charge pump.
A conventional charge pump used in a flash memory device is driven by an oscillator circuit that is designed to produce a stable frequency output that is relatively unaffected by normal variations within the specified temperature and supply voltage ranges. The charge pump is also designed to provide a minimum at a slow process corner, that is a worst case scenario for the process variations that may occur during fabrication of the device.
There are many variables that may be involved in determining a process corner. Examples are junction doping profile (hard or soft), gate oxide (thin or thick), contact resistance (high or low), and gate length (long or short). Each of these variables has an impact on the overall performance of a circuit. A hard (abrupt) junction doping profile, thin gate oxide, low contact resistance and a short gate contribute to a fast process, whereas a soft junction doping profile, thick gate oxide, high contact resistance and a long gate contribute to a slow process.
Although a charge pump oscillator may be designed for a slow process corner and worst case operating conditions (e.g., low supply voltage and high temperature), the fabricated device is usually better than worst case, and is used under operating conditions that are not extreme. The result of having a “better than expected” device operating under mild or favorable conditions is that the output of a flat oscillator exceeds that required to meet the performance specifications of the flash memory as a whole, and power is wasted. What is needed is an oscillator that is capable of producing a uniform power dissipation in a flash memory device over a range of process and operating environment variables.